A particularly advantageous application of the invention lies in the field of audio equipment, in particular audio equipment having an incorporated microphone, such as car radios provided with a “hands-free” telephony function. FR 2 890 513 A1 (Parrot) describes such a car radio having a front face that includes two microphones suitable for picking up the voice of the driver for telephone conversation, and possibly also for voice commands.
In this context, the invention seeks more particularly to improve listening to music programs, regardless of whether they are produced by a source of the audio equipment operating in “streaming” mode, by a signal applied to the “line-in” input of the appliance, or by radio broadcast signals.
A user listening to such programs, e.g. a person present in a motor vehicle fitted with a “hands-free” car radio, is strongly influenced by the sound level of the ambient noise surrounding the user. It is therefore desirable to be able to adapt the sound volume of the useful audio signal to the volume of ambient noise by automatically increasing or decreasing volume as a function of corresponding variations in ambient noise.
In this respect, the presence of a microphone in the audio equipment is particularly appropriate since, in addition to its main function dedicated to “hands-free” communication, the microphone can also perform another function, namely that of measuring the level of ambient noise and variations therein in order to deduce automatically therefrom the gain that ought to be applied to the audio signal, and to adjust the useful volume to a level that provides the user with the same listening comfort regardless of the sound environment surrounding the user.
Nevertheless, the use of a microphone as an ambient noise sensor presents a difficulty associated with the fact that the microphone is capable also of sensing the echo of the music played back by the loudspeakers and the voices of people present in the vehicle.
These two elements should not be considered as ambient noise. For example, it will be understood that if the level of the music increases in the vehicle, the power of the sound signal sensed by the microphone will increase correspondingly by the echo effect. If this increase were to be considered as noise, then the gain applied of the audio signal would itself be increased, thereby simultaneously increasing the level of the music and thus once more increasing the power of the echo sensed by the microphone. This can lead to a diverging loop.
The same applies to the voices of people. If a conversation is taking place on board the vehicle, the corresponding increase in the power sensed by the microphone must not be considered as an increase in ambient noise that might lead to an automatic increase in the gain of the audio signal, since that increase would impede the conversation (already disturbed by the ambient noise), which would be contrary to the desired objective.
Concerning the problem associated with acoustic echo, systems are known in the state of the art for performing automatic volume control while eliminating acoustic echo from the sound signal picked up by the microphone.
For example, U.S. Pat. No. 6,868,162 A1 describes such an automatic gain control system in which the audio signal is processed by an adaptive filter, and is then subtracted from the signal detected by the microphone, with the resulting error signal providing an estimate of the ambient noise.
In another system, proposed in WO 01/89184 A1, an estimate for the current noise value is established periodically, directly from the signal delivered by the microphone after subtraction of the echo-canceling signal.
Nevertheless, it should be observed that implementing an echo canceller requires significant computation resources for the processor that is processing the signal. Thus, although using a device of that type leads to a better estimation of ambient noise, it should be considered as being optional and suitable for use only by equipment having sufficient computation resources.
Concerning the problem associated with the voices of people, there is nothing in the present state of the art that provides a solution for eliminating those voices from the signal sensed by the microphone.